This invention relates to data communication networks, e.g., so-called local area networks ("LAN"s).
Such networks have come into widespread use to allow computers, peripheral user devices and stations, and other data processing and communication devices to communicate among themselves.
One type of network connects each device to central switching circuitry that functions as a switchboard, connecting each transmitting device with its intended receiving device as free lines become available.
Another type of network uses the so-called packet switching technique, as described, e.g., in Metcalfe et al. U.S. Pat. No. 4,063,220. Each device is connected to a common network (e.g., a coaxial cable bus) through a network interface unit that controls the access of its associated device to the network; if the interface unit senses that the network is free of carrier energy it allows its associated device to transmit a packet of bits consisting of a preamble, address information, and message data. Due to propagation delays on the line, two or more such interface units may permit their associated devices to initiate transmissions that will overlap in time, and create a "collision" on the network. A transceiver has "collision detection" circuitry for detecting collisions and for sending a collision signal to all interface units, which abort any transmissions they might have begun upon receipt of such signals. A network design incorporating such a collision detection system is called a carrier sense multiple access with collision detection (CSMA/CD) system.
A commercial embodiment of such a CSMA/CD system has been marketed by Xerox under the trademark, Ethernet. This embodiment uses network interface units (NIU's) that conform to the IEEE 802.3 standard and that have four input/output lines (each typically a twisted pair of wires) that, respectively, allow the interface unit to transmit packets, receive packets, receive collision signals, and supply power. The physical connection of these lines to the coaxial bus is made by a "vampire connector", a mechanical device that makes contact with the coaxial sheathing and also pierces the sheathing to make contact with the central conductor.
Another type of network design, called carrier sense multiple access with collision avoidance (CSMA/CA), prioritizes attempted transmissions and allows only the then highest priority transmission to be transmitted, thereby avoiding any actual collisions within the network.
In one embodiment of such a CSMA/CA, as described in, e.g., "Systems View of Token-Ring Local Area Networks," paper by Gary L. Wood given at TI-MIX Symposium: Communicating and Networking, Apr. 5-8, 1983, a number of devices with interface units are wired to a central unit in a logical loop. A "token" signal moves continually through this loop and determines what unit may transmit at any time, thus avoiding collisions in the system. Loops may be connected to form larger loops operating on the same principle.